With recent developments in the nuclear power industry, stable operation of the nuclear power plants has become more important than ever before. Among several factors involved in improving safety in the nuclear power plants, stable and effective management of spent nuclear fuel is extremely important. Since the nuclear fuels (enriched uranium) used in a nuclear power plant should be periodically replaced with new nuclear fuels when the use period expires, the spent nuclear fuel should be also periodically discharged. Since such spent nuclear fuels emit radiation, shielding for radiation emitted from a spent nuclear fuel transportation cask is critical for human beings as well as the environments.
Throughout the world, spent nuclear fuel can be stored by using two methods, which are the dry and wet storage methods. For example in Korea, spent nuclear fuels are temporarily stored in storage sites in nuclear power plants using both the wet method and the dry method. In order to store spent nuclear fuel in storage sites for a predetermined period of time, or store spent nuclear fuel in intermediate storage facilities, permanent processing facilities, re-processing facilities or the like, the spent nuclear fuel needs to be transported using a transportation cask. In this case, transportation casks for such spent nuclear fuel need to have a neutron shielding material provided therein. As such a neutron shielding material, in general, a shielding material having relatively high hydrogen content, such as an epoxy, a high-density polyethylene, polystyrene, water, ethylene glycol or the like, may be used.
The radiation shielding material should be able to allow for the amount of radiation emitted from spent nuclear fuel transportation casks to be significantly reduced for the safety of humans and to prevent structural materials or equipment components from being damaged or contaminated. In particular, since neutrons generated from spent nuclear fuel have high energy and high penetrating powder, the research for development of an efficient neutron shielding material capable of stably and definitively shielding high energy neutrons is making progress.
The neutrons as described above are classified as fast and thermal neutrons, depending on the amount of energy thereof. When a fast neutron collides with a light element such as hydrogen, it loses energy (speed), and thus becomes a slow neutron, and slow neutrons as above are finally absorbed by a material having a large slow neutron absorption cross sectional area, such as boron, lithium and gadolinium, provided in the neutron shields in the spent nuclear fuel transportation casks.
Therefore, as a fast neutron shielding material, a material obtained by mixing a shielding material should be able to absorb a thermal neutron in a polymer compound having a high hydrogen concentration, that is, a polyolefin-based thermoplastic resin such as ethylene glycol, polyethylene, or the like, a thermosetting resin such as an unsaturated polyester resin or the like, or a resin such as an epoxy resin or the like, may be used.
Meanwhile, primary and secondary gamma rays are generated by the spent nuclear fuels as described above. Secondary gamma rays may be shielded by a mixture of a high density metal powder, and the high density metal powder is added to a mixture thereof, such that a shielding material density is generally 1.6 g/cm3 or more.
In addition, the development of a neutron shielding material is able to maintain neutron shielding capabilities, even in a case in which a fire breaks out has progressed. In this connection, as a flame retardant, a neutron shielding material including an aluminum hydroxide powder, a magnesium hydroxide powder or the like, mixed therewith, has been proposed.
As described above, as the added neutron absorbing particles, a neutron shielding powder having an average powder particle size of 10 to 200 μm, in consideration of dispersion properties in a polymer resin, a matrix, and shielding properties against neutrons has been used. However, since the size of such a neutron shielding material, a high density metal powder or a flame retardant particle is relatively large, on the level of micrometers or more, there are the possibilities of the occurrence of radiation leak or the particles as described above acting as impurities, thus acting as a factor deteriorating physical properties of a composite material.
It is therefore necessary to increase the possibility of collisions between radiation and a neutron shielding material having neutron absorber and high density metal particles, and to this end, a method of increasing radiation shielding efficiency by miniaturizing particles to be used may be considered.
However, in a case in which a nanoscale radiation shielding material is used, although advantages in that a radiation shielding function and physical properties of a composite material are improved and the like, are provided, since such a nanoscale shielding material has high degree of surface energy to increase viscosity of a polymer, it may be difficult to perform roll mixing milling in a matrix resin, and since nanoscale particles are in an unstable state and thus have a nature in which particles are agglomerated with each other, it may be difficult to allow nanoscale particles to be uniformly dispersed in a polymer resin.
In general, in a case in which nanoscale powder particles are dispersed in a polymer resin, it may be important to prevent powder particles from being agglomerated and also improve adhesion between the fine particles and the resin, and to this end, a method in which a particle surface is chemically treated by using a surfactant may generally be used.
However, performance of a chemical surface treatment using a surfactant may be complicated, a surfactant itself may act as a foreign object, and process costs may be unfeasible due to high costs incurred in producing a polymer (epoxy) composite material.
The present inventors found that a particle size of the radiation shielding particles in a shielding body has a significant influence on radiation shielding capabilities, and in detail, when the nanoscale radiation shielding particles are included in a shielding body, the possibility that incident radiation will collide with the nanoscale radiation shielding particles increases, so thereby improve radiation shielding effects, and on this basis, filed an application for a patent (Korean Patent Laid-Open Publication No. 2010-0047510, this patent was divided, one is registered and the other went to the court) related to a radiation shielding material for allowing for nanoscale radiation shielding particles to be uniformly dispersed in a polymer base or the like. The patent document described above also discloses a technology for physically surface-processing nanoscale particles with a surfactant having high affinity with a polymer resin to melt into and be mixed with the polymer resin during a process of producing nanoscale particles through a ball milling process in order to obtain uniform dispersion of nanoscale particles.